1. Field of the Invention
The invention lies in the field of signal reception, and relates to an automatic gain control method and device for receiving circuits.
It relates to the signals transmitted via a transmission channel affecting, in a related way, the DC component of the transmitted signal and the useful signal. This is particularly the case in the field of video communications networks using transmission by optical fibre.
2. Discussion of the Background
Automatic gain control methods for receiving circuits of transmission equipment make it possible to overcome the losses of the transmission channel by monitoring the level of the signal at the equipment output. The overall gain of the system can then be considered as constant. They are usually based on a measurement of the output signal from these receiving circuits so as to act on a gain control of an amplifier making up these circuits so as to compensate for the variations in the signal due to the losses of the transmission channel. It is thus possible to achieve gain slaving by virtue of an error signal derived by comparison of supposedly stable characteristics in the transmitted signal, which are measurable and recognizable in the received signal, with a reference. These characteristics are, for example, the amplitude of synchronization pulses or the peak-to-peak amplitude of the transmission signal. If the useful signal to be transmitted does not possess such characteristics, they can be added artificially to the signal on transmission. Hence a reference signal can be superimposed on the useful signal, for example a signal of known amplitude and the frequency of which is outside the spectral limits of the useful signal, called pilot signal. It may also be envisaged, as described in French Patent Application No. 2 621 753 filed by the Applicant, to add a constant well-defined DC component to the signal to be transmitted in the case in which the latter is devoid thereof, or in the case in which the original DC component is not well defined or stable, the latter then first of all being discarded.
However, the measurements of these reference signals are often tricky, the extraction or the measurement of their intrinsic characteristics being disturbed by the noise of the link. The precision of the gain slaving depends on the transmission quality. In the event of a superposition of a pilot signal in the transmission circuits, an improvement in the signal-to-noise ratio can certainly be obtained by increasing the amplitude of the pilot signal, but then the useful dynamic range of the transmission channel is reduced and the stray intermodulation signals are also increased; an enhancement can also be obtained by refining the filtering in the receiving circuits, but a problem then appears of design of the filter and of stability of the pilot frequency. Moreover, the receiving circuits, the gain control of which is based on a measurement of the level of a transmitted signal, are particularly sensitive to jamming or other interference which desensitizes these receivers. Finally, the reference signals, due to their characteristics, are often outside the passband of the circuits receiving the useful signal; closed-loop slaving of gain of these circuits on the basis of these signals cannot then be achieved.